Such a pulse radar ranging system is known from U.S. Pat. No. 4,521,778 and U.S. Pat. No. 4,132,991.
Pulse radar ranging systems provide distance or level measurement based on the direct measurement of the running time of microwave pulses transmitted to and reflected from a target, e.g. the surface of a fill material in a container. As the running time for distances of a few meters is in the nanosecond range, a special time transformation procedure is required to enable these short time periods to be measured. The microwave pulses are transmitted to the target at a repetition rate or transmit clock frequency which is given by a transmit clock generator. In a signal mixer, the received echo pulses reflected from the target are sampled by cross-correlation with sampling pulses of the same shape as the transmit pulses but at a sampling clock frequency slightly lower than the transmit clock frequency. The cross-correlation and subsequent integration or low-pass filtering leads to an intermediate frequency (IF) signal corresponding to the received echo pulses but time-expanded relative thereto by a factor T1/(T1−T2), where T1 is the transmit pulse repetition period and T2 is the sampling period. The time-expansion allows for amplifying, digitizing and further processing of the echo pulses with standard techniques.
One of the sources of errors in pulse radar ranging systems is the temperature drift. Due to variation of the parameters of the semiconductor devices of the pulse radar, the level measurement result will change over the specified temperature range without a real change of the measured level.
From US 2008/0036649 A1 a pulse radar ranging system is known, where a controllable switch, depending on a control signal, either conveys the transmit pulses to the antenna to be transmitted to the target or to a calibration module, preferably a delay line of known delay and terminated with a pulse reflecting impedance mismatch. Thus, the known pulse radar ranging system has two operating modes, wherein a measuring mode is periodically interrupted by a calibration or diagnostic mode. In the calibration or diagnostic mode the equivalent of a reference distance, given by the delay line, is measured. For calibrating the pulse radar, corrections are calculated from the measured reference and are applied to the target measurement results. For diagnostic purposes, the value measured from the reference is checked against an acceptable range. The performance of the switch may impose limitations with respect to frequency range or noise on the measurement function and the calibration or diagnostic function.